Eyepieces

An eyepiece, or ocular lens, is a type of lens that is closest to the eye

when someone looks through the optical device.

The eyepiece is placed near the focal point of the objective to magnify this

image. The amount of magnification

depends on the focal length of the eyepiece.

The lens or combination of lenses in an optical instrument (microscope, telescope, etc.) through which the observer views the imag

e formed by the objective. The most common eyepieces are composed of two single lenses or two doublets: the lens or doublet nearer the eye is called the eye lens and the one nearer the objective is called the field lens.

 The role of the eyepiece is to magnify the image and to reduce the aberrations of the i

mage formed by the objective.

Negative eyepiece :Eyepiece made up of two lenses, in which the first principal focus of the eyepiece lies between the two lensessuch as in a Huygens' eyepi

ece.

Positive eyepiece: Eyepiece made up of two lenses in which the first principal focus of the eyepiece lies in front of the 

field lens, such as in a Ramsden eyepiece.

Types of Eyepieces1.Ramsden Eyepiece2.Huygens Eyepiece

Ramsden EyepieceAn eyepiece consisting of two plano-

convex crown-glass lenses of equal focal length, placed with the convex sides facing each other andwith a separat

ion between the lenses of about two-thirds of the focal length of each.

Huygens Eyepiece• A telescope eyepiece consisting of two

planoconvex lenses separated by a distance equal to half the sum of their focal lengths, which are in the ratio of three to one, and oriented so that their curved surfaces face the incident light..

Comparison of Ramsden & Huygens Eyepiece

Ramsden Eyepiece Huygens Eyepiece

Ramsden’s eyepiece is a positive eyepiece. The image formed by the objective lies in front of the field lens. Therefore , crosswires can be used.

Huygen’s eyepiece is a negative eyepiece. The image formed by the objective lies in between the two lenses. Therefore crosswires cannot be used.

The condition for minimum spherical aberration is not satisfied. But by spreading the deviations over four surface ,spherical aberration is minimized.

The condition for minimum spherical aberration is satisfied.

It does not satisfy the condition for achromatism but can be made achromatic by using an achromatic doublet as the eye lens.

It satisfies the condition for achromatism.

It is achromatic for only two chosen colors. It is achromatic for all colors.

The other types of aberration are better eliminated. Coma is absent and distortion is 5% higher.

Other aberration like pincushion distortion are not eliminated.

The eye clearance is 5% higher. The eye clearance is too small and less comfortable.

It is used for quantitative purpose in microscopes and telescopes.

It is used for qualitative purpose in microscopes and telescopes.

Ramsden Eyepiece Huygens Eyepiece

Its power is positive Its power is positive

The two principle planes are crossed. The two principal planes are crossed

It can be used as a simple microscope because the first principal plane lies to the left of the field lens and the focal plane is real.

It cannot be used as simple microscope because the first focal plane lies to the right of the field lens and the focal plane is virtual.

The nodal points coincide with the principal points.

The nodal points coincide with the principal points.

Aberration

The departures of real images from the ideal images in respect of actual

size shape and position.They are inevitable consequences of

the laws of refraction at spherical surfaces.

Types of aberration

Monochromatic Aberration:The defects due to wide angle

incidence and peripheral incidence.It occurs with monochromatic light.

Chromatic Aberration:It occurs due to dispersion of light.

Types of Monochromatic

aberrationSpherical aberration

ComaAstigmatism

Curvature of fieldDistortion

Spherical Aberration Spherical aberration comes from the

spherical surface of a lens.The further away the rays from the lens

center, the bigger the error is. The image is improved if the image

plane move closer to the lens to find optimal spot

size.

Coma•Arises from off-axis object points.• The transverse magnification is a

function of ray height• The resulting pattern is like a comet.

Astigmatism•In optical design, the vertical plane

is general called the “tangential plane”

• The “sagittal plane” is the plane at right angle to the trangential plane and containing the principle ray.• Astigmatism results in different focusing power to the tangential

and sagittal plane.

Distortion• All points in the object plane are

imaged to points in image plane.• Distortion arises when he

magnification of off axis image is a function of the distance to the lens

center.With distortion

Corrected

Reference:Text book of optics

Fundamentals of opticshttp:/aberration/wikipedia/

encyclopediahttp:/types of aberration/

ppt/encyclopedia/freedownload.html

http:/eyepiece/wikipedia/encyclopedia.html

http:/wikipedia/encyclopedia/types of eyepiece.html

Done By A.K.RAAHA SUTHA(13PHY040)

D.DEEPIGA(13PHY010)